1. Field of the Invention
This invention relates to apparatus for damping power oscillations in electric power systems. More particularly it relates to a series compensator which can insert both real and reactive impedance into a transmission line to dampen the power oscillations.
2. Background Information
Power oscillation frequently occurs in electric power systems due to disturbances, such as transmission line faults, line and load switchings, equipment failures and other events causing rapid system changes. Such power oscillation has the undesirable effect of limiting the maximum transmittible power in the system. U.S. Pat. No. 5,198,746 discloses a solid-state series compensator that injects a controllable, 60 Hz voltage in quadrature with the transmission line current. This injected voltage has the same compensating effect as that obtained with a variable capacitor, or inductor, connected in series with the line. The damping of the power oscillation is achieved by appropriate modulation of the effective 60 Hz reactive impedance that the compensator injects in series with the line. That is, when the power in the line is increasing, as a result of acceleration of the power generator at the "sending-end" of the line and the consequent increase of the transmission angle, the capacitive impedance of the compensator is increased by increasing the magnitude of the inserted 60 Hz voltage that lags the line current by 90 electrical degrees, in order to increase the degree of series compensation and thereby the transmittable power. Conversely, when the power in the line is decreasing, as a result of the deceleration of the "sending-end" generator and the consequent decrease in the transmission angle, the inserted voltage is made to lead the line current in order to create, in effect, an inductive output impedance and thereby to increase the overall inductive impedance of the transmission line.
The series compensator of U.S. Pat. No. 5,198,746 utilizes a dc to ac converter to generate the quadrature voltage. The ac terminals of the inverter are connected in series with the transmission line through a coupling transformer. A capacitor connected across the dc terminals provides the dc input voltage for the converter. As the converter is theoretically only exchanging reactive power with the transmission line, there is no power drain on the capacitor. However, there are in reality losses in the switching circuits of the inverter. The real power needed to make up for these losses is provided by generating the compensating voltage at a phase angle slightly less than 90.degree..
U.S. Pat. No. 5,343,139 discloses a generalized power flow controller for controlling the flow of power through the transmission line on a sustained basis. This power flow controller also utilizes a de to ac inverter which injects a voltage in series with the transmission line voltage. However, the phase angle of the injected voltage relative to line current is not limited to substantially 90.degree. as in the case of the compensator of U.S. Pat. No. 5,198,746, but is controllable to any phase angle between 0 and 360.degree.. The particular phase angle and the magnitude of the injected voltage may be selected to adjust any or all of the transmission line effective impedance, the effective phase angle between voltages at the two ends of the transmission line, and transmission line voltage magnitude. This results in the adjustment of both real and reactive line impedance. Real power needed to adjust the real component of impedance is provided to the dc to ac converter by an ac to dc converter having its ac terminals connected in shunt to the transmission line and its dc terminals connected to the dc terminals of the compensator converter through a de link which includes a capacitor. The response of this flow controller is rapid enough that it can provide dynamic control of power flow, and it is also suitable for damping oscillation which may develop in the power system. However, this equipment designed primarily for power flow control, is more complex and expensive than the series compensator described in U.S. Pat. No. 5,198,746.
There is a need for improved compensator for damping oscillations in electrical power systems.
More particularly, there is a need for such an improved compensator which can provide real power modulation as well as reactive power modulation to damp out power system oscillations.